Multi-user communication (multiplex communication) between a base station and a plurality wireless communication terminals (hereinafter, referred to as terminals) is discussed. Uplink multi-user communication is represented as UL-MU (UpLink Multi-User) communication, and downlink multi-user communication is represented as DL-MU (DownLink Multi-User) communication.
As the multi-user communication, frequency multiplexing communication is known according to which different frequency components for each wireless communication terminal (hereinafter referred to as terminal) are used as communication resources and transmissions to a plurality of terminals and receptions from a plurality of terminals are simultaneously performed. Here, Orthogonal Frequency Division Multiple Access (OFDMA) scheme is considered where the frequency components are defined as resource units each including one or a plurality of subcarriers, and the resource units each are used as a smallest unit of the communication resource, and transmissions to the plurality of terminals or receptions from the plurality of terminals are simultaneously performed. The simultaneous transmissions from the base station to the plurality of terminals correspond to downlink OFDMA (DL-OFDMA) transmission and the simultaneous transmissions from the plurality of terminals to the base station correspond to uplink OFDMA (UL-OFDMA) transmission. The DL-OFDMA is an example of the DL-MU, and The UL-OFDMA is an example of the UL-MU.
In a case of the UL-OFDMA, it may be considered that in order to match timings of uplink transmission, a trigger frame specifying terminals that are to be subjected to the UL-OFDMA and resource units allocated to the terminals is transmitted from a base station. This method has problems that in a case such as where the specified terminal is transited to a sleep mode, or the specified terminal has no uplink transmission request, the resource unit allocated to the specified terminal is not efficiently used and a usage efficiency of a communication resource is decreased.
There is another method in which the trigger frame does not specify any terminal, but specifies only resource units that are to be used. The method may include a case where a part of the resource units may be specified with respect to a terminal, but remaining resource units are specified with respect to no terminal. In any cases, the terminal to which no resource unit is allocated, of the terminals receiving the trigger frame, selects a resource unit randomly from the resource units specified with respect to no terminal (also referred to as “STA-unspecified RU”) to use the selected resource unit. The trigger frame specifying the STA-unspecified RU may be called a trigger frame for random access.
Examples of the method selecting the resource unit randomly include the following method. Every time the trigger frame for random access is received, a random number (backoff count) selected from a contention window (CW) for random access is decremented by a value corresponding to the number of STA-unspecified RUs. If the decremented backoff count becomes below or 0, an access right with respect to the STA-unspecified RU is obtained, the resource unit is selected randomly from STA-unspecified RUs, and the frame is transmitted using the selected resource unit. Note that the CW for random access is different from a contention window used for deciding a backoff time to carrier sense in CSMA/CA.
In the above method, if plural terminals simultaneously obtain the access right, these terminals may select the same STA-unspecified RU to transmit their frames. In this case, the frames transmitted from these terminals collide at an access point, where the frames cannot successfully be decoded. Each of these terminals does not receive an acknowledgement response from the access point, and thus, determines that the frame transmitted by the terminal is not successfully received. In this case, these terminals select a random number from the CW for random access which is newly set by way of receiving the next trigger frame for random access and perform the same process, but at this time, how the CW is selected again is not clear. Selecting the CW for random access under the same condition as the terminal having succeeded in the transmission (e.g., terminal not selecting the same resource unit as other terminals) may lack fairness. In addition, as for also the terminal from which the frame is successfully received at the access point, how the CW is selected again is not clear. In this way, this method lacks a protocol for repeatedly using the trigger frame for random access.
Particularly, in a case where the trigger frame for random access is used to collect a UL-MU allocation request (uplink transmission request) from the terminals, the terminal having failed in the transmission from which the request is not received by the access point is not selected as a terminal to be subjected to the UL-MU. On the other hand, the terminal having successfully transmitted the UL-MU allocation request may be likely to be selected in accordance with the request as a terminal to be subjected to the UL-MU (given an opportunity to transmit by way of the UL-MU). Therefore, selecting the CW for random access under the same condition as the terminal having succeeded in the transmission may lack fairness.